The present invention relates to mechanisms for providing offset and gain correction of the output from a floating-point amplifier; and more particularly, to such a correction mechanism specifically adapted for use in amplifier circuits of computed tomography (CT) imaging apparatus.
In a computed tomography system, an X-ray source projects a fan beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the "imaging plane". The beam is transmitted through an object to be imaged and then impinges upon an X-ray detector array oriented within the imaging plane. The detector array is formed of a plurality of individual elements with each element measuring the intensity of the radiation transmitted from the source to that element. The intensity of the transmitted radiation is dependent upon the attenuation of the X-ray beam by the object.
The X-ray source and detector array in a common CT system are rotated on a gantry within the imaging plane and around the imaged object so that the angle at which the fan beam intersects the object constantly changes. As the gantry rotates, a number of X-ray projections forming a projection set are acquired. Each projection is made up of the intensity signals from the detector elements as they travel over a small angle of gantry rotation centered about a projection angle.
The acquired tomographic projection sets typically are stored in numerical form for computer processing to reconstruct a slice image according to array processing algorithms known in the art. A set of CT projections may be transformed directly into an image by means of a fan beam reconstruction technique or the intensity data of the projection may be sorted into parallel beams and reconstructed according to parallel beam reconstruction techniques. The reconstructed tomographic images then are displayed on a video monitor or converted into a film record by means of a computer controlled camera.
In conventional CT systems, the signal from the detector array is amplified and digitized in a unit referred to as a data acquisition system (DAS). The data acquisition system is composed of two primary components: a floating-point amplifier and an analog to digital converter. Both of these devices may introduce errors into the signal from the detector array. For example, the DAS amplifier may introduce a signal offset which varies with the different gain settings of the amplifier, as shown in FIG. 1A. This figure graphically depicts the amplifier transfer function, i.e. input versus output voltage levels, for two gain settings G.sub.1 and G.sub.2 of the floating-point amplifier in the DAS. Each of these different gain levels has a separate offset error which corresponds to the amount that the transfer function is displaced along the vertical axis from the origin. A conventional technique for compensating offset errors collects data when the X-ray beam is off and the detector signal is relatively small. As a result, the offset error data is acquired at the highest possible gain level of the amplifier (e.g gain G.sub.1) and the resultant offset value is used thereafter to compensate not only signals produced by the amplifier at this gain level but at all other gain levels. In general, the offset at the other gains is not the same and a single offset correction actually introduces errors by shifting the transfer function of other levels (e.g. G.sub.2) a constant amount as shown in FIG. 1B. In this case, the offset correction has been set to compensate for the amplifier errors at a gain G.sub.1, but has not eliminated the offset error at gain G.sub.2.
Likewise, each gain level of the programmable amplifier can have a unique departure from its ideal transfer function due to gain error. Each gain level G.sub.1 and G.sub.2 in the example provide different deviations from their ideal transfer functions I.sub.1 and I.sub.2 respectively, as shown in FIG. 2A in the absence of offset errors. Previously, a single gain correction factor was determined from the error at the highest gain level. This single correction factor Gc compensates for the error at G.sub.2 of the floating-point amplifier to bring its transfer function into coincidence with its ideal I.sub.2 as shown in FIG. 2B. However, the single gain correction factor can actually increase the error at another gain setting (G.sub.1) as illustrated.
Furthermore, previous CT systems used the array processor to reconstruct the image from the set of projections. The array processor also was saddled with the task of performing the error correction as well. This degraded the performance of the array processor as it required continual interruption of the image reconstruction to apply offset and error correction to the incoming projection data. Using the array processor in this manner increased the time required between data collection and image presentation.